Optical Coherence Tomography (OCT) is an imaging technique that allows micrometer scale imaging over short distances. It is based on a novel extension of optical coherence domain reflectometry (OCDR), a one dimensional optical ranging technique used for investigating the properties of fiber and waveguide based optical devices. OCT relies on the coherent interference between a reference wave and a probe wave to measure the distances and thicknesses of a material. Extensions exist to measure flow, refractive indexes, and the polarization properties of samples under test.
Due to its non-contact, non-invasive and non-destructive nature, OCT has found extensive application in the biomedical imaging field, ranging from ophthalmology to neurology, dermatology, dentistry, developmental biology, urology, and gastroenterology as an in-vivo diagnostic tool. The properties of OCT imaging also have applications many non-biomedical applications ranging from dimensional metrology, material research and non-destructive testing, over art diagnostics, botany, microfluidics, to data storage and security applications. Currently developing OCT techniques have high potential for future applications both inside and outside the scope of the biomedical field.
Generally, optical coherence tomography systems can be divided into three main types depending on how the depth information is obtained. If the depth ranging is obtained by varying the path length difference between the sampling and reference arms in time, the configuration is referred to as time-domain optical coherence tomography (TD-OCT). TD-OCT systems are usually implemented with a movable mirror in the reference arm of the interferometer to provide the path length difference. When depth information is extracted from the spatial variation of the interferometric signals spectral components the system is known as spectral domain optical coherence tomography (SD-OCT). The temporal variation of the spectral components is known as swept source optical coherence tomography (SS-OCT). Due to the spectral nature of both SS-OCT and SD-OCT, they are both also known under the title of Fourier domain optical coherence tomography (FD-OCT).
Examples of the architectures are shown schematically in FIG. 1 to FIG. 4. The FD-OCT architectures offer increased speed and theoretical sensitivity when compared to TD-OCT systems. FIG. 1 is a schematic block diagram of a basic optical coherence tomography system to which the proposed system is applicable. FIG. 2 is a schematic block diagram of a time domain optical coherence tomography system as described in the prior art. FIG. 3 is a schematic block diagram of a spectral domain optical coherence tomography system as described in the prior art. FIG. 4 is a schematic block diagram of a swept source optical coherence tomography system as described in the prior art
In the above prior art arrangements, it is advantageous to fully decode the output signal in order to have both the phase and amplitude information of the received signal. In Doppler OCT, multiple measurements of the phase can be used to increase the velocity sensitivity. In FD-OCT systems the lack of phase information leads to a known complex conjugate ambiguity, where the positive and negative spatial frequencies of the signal cannot be separated. This complex conjugate ambiguity results in image contamination with double images, undesirable autocorrelation terms and wasted resources (pixels in SD-OCT and time in SS-OCT) due to a reduction in imaging depth. See, for example, “Demonstration of complex-conjugate-resolved harmonic Fourier-domain optical coherence tomography imaging of biological samples”, Vakhtin et al., Applied Optics, Vol 46, No. 18, p 3670.
As the opto-electronic conversion process relies on square law detectors, the phase information in the interferometric signal is lost upon detection and only the real part of the signal is obtained. This results in a complex conjugate ambiguity that cannot be resolved or removed via post processing, necessitating the use of the full complex signal to remove the ambiguity.
Various methods reported that allow for the recovery of the full complex signal include the use of polarization quadrature encoding, phase stepping, N×N couplers (with N≧3) and synchronous detection. In polarization quadrature encoding, orthogonal polarization states encode the real and imaginary parts of the signal, but the setup is complex and the signal suffers from polarization fading as the reference and sample arms approach a quasi-orthogonal state. Phase stepping requires the reference mirror to be sequentially displaced encoding the real and imaginary components in time. However, phase stepping is not instantaneous and sensitive to small drifts in the interferometer. Synchronous detection requires the signal is mixed in a heterodyne arrangement where the complex signal is generated electronically (as opposed to the previously mention methods where the signal is generated optically). Relying on an electronic carrier frequency, synchronous detection is unsuitable for homodyne detection systems, such as in FD-OCT setups. The use of couplers (N×N with N≧3) couplers generate signals with a non 180° phase shift from which the complete complex signal can be calculated by the cosine rule.
Polarization fading is also a problem for most interferometric systems. The main methods to avoid its effects are either to recover the full polarization information or through strict control of the polarization states as the light beams pass through the interferometer. This often requires the use of polarization beam splitters, polarization maintaining fibers or a doubling of the receiver structure (one for each polarization).